686 THE CEREBRUM 



of the retinae. In many of these cases, however, a tertiary type of 

 degeneration frequently results which involves certain neurons which 

 are not directly affected by the primary lesion. This spreading 

 gives rise to " sympathetic " effects, so that bilateral defects in vision 

 may be obtained in spite of the fact that the original injury is con- 

 fined to, say, one of the optic nerves and should, therefore, have pro- 

 duced blindness in only the corresponding eye. 



Visual Association. Upon genetic grounds it must be granted that 

 the optic nerves are really not peripheral nerves at all, but true cerebral 

 tracts, bearing a close resemblance to the lemniscus and other systems. : 

 Hence, the retina must be regarded merely as an exposed feeler of the 

 nervous system which is excited by the .ethereal rays of light entering 

 its substance. The impulses here generated are transferred to central 

 parts over neurons, the cell-bodies of which are situated in the retinae. 

 It is true, however, that the optic nerves also embrace a small number 

 of centrifugal conductors which end in arborizations around certain 

 elements of the retinas. The function of these fibers is not known. 

 We have previously seen that the centripetal fibers of this tract con- 

 nect with the superior colliculus, lateral geniculate, and thalamic 

 nuclei, and that the psychovisual centers in the occipital realms of 

 the cerebrum are more directly reached by way of the thalamo- 

 geniculate bodies and the occipitothalamic radiations. In the course 

 of the development of this cortical area, the importance of the lower 

 visual centers formed by the aforesaid masses of gray matter, dimin- 

 ishes gradually. In the higher animals, the latter retain merely the 

 function of ordinary relay stations for reflex action, while visual per- 

 ception and memory are concentrated in the cortical area. In the 

 simpler forms, such as the fish, these lower centers form the terminal 

 stations of the optic tract and must, therefore, be capable to mediate 

 in addition the psychical processes connected with vision. 2 It may 

 be concluded, however, that the psychical activity of these animals 

 is at best extremely rudimentary. 



The psycho-visual area is composed of two fields, one being re- 

 stricted to visual perception and the other to visual memory. Having 

 reached the visual sphere, the retinal impulses are transferred to con- 

 sciousness as perceptions which are then relegated to the memory field 

 by way of association fibers. Stress has been placed upon the fact that 

 the visual center cannot be restricted to a narrow sphere, although 

 Henschen 3 has stated that the visual paths of man terminate around 

 the calcarine fissure on the mesial surface of the cerebrum. In support 

 of this contention it has been mentioned that the examination of the 

 brain of Laura Bridgman, 4 the blind deaf-mute, has shown decided 



1 Parker, Am. Nat., xlii, 1908, 601. 



2 Harris, Brain, xxvii, 1904, 106; also: Vincent, Jour. Animal Behavior, ii, 

 1912, 249. 



3 Brain, xvi, 1893, 170. 



4 Donaldson, Am. Jour, of Psychol., 1892, 4. 



